A New Construction window includes nailing fins used to fasten the window in place and to flash it, helping to make it airtight and waterproof. This usually requires installation of new interior woodwork, and sometimes touch-up painting of the interior drywall. It also can require removal of the siding around the window, or new wide trim installation, depending on circumstances.

A replacement window fits inside of the current window frame, leaving the original exterior & interior window frame & trim in place. This is a quicker and less expensive method of replacing windows. But it can also leave existing problems in place. IE, if there is air leakage between the existing window jamb and rough opening, a replacement window will not solve it. If there is rotted wood, the rotted wood often gets covered by aluminum cladding but is not necessarily removed/repaired.

There are some good companies who do a good job of replacement winodws and I'm not putting down the good ones. There are also some really bad ones, who sell throw-away window products that are a good stop-gap measure. IE, replace 2 worst windows while you wait to replace the entire house full of windows and do new siding and energy improvements all at once.

For this reason, my company almost always installs New Construction windows and usually does so at the same time as a whole-house improvement with better insulation, air sealing, and new siding-soffit-fascia as well. It's the best practice if its what your home needs and you can afford to do it right. I suggest saving up to do it right, even if you need to live with a deteriorating product a little longer. It's better for your home in the long run, and better for our nation's housing stock as well!

The photos below show before, during, and after-- a new construction window, without installing new siding.

Showing some rough-construction photos too of the type of damage that we often discover and fix. How we install peel-n-stick flashing to make a window replacement be "as-good" as a new home construction. If we're replacing the siding, we can literally do so. WIthout replacing siding, we're limited to the amount of surface we have exposed.

When installing replacement windows, the existing window frame remains and the replacement window is installed inside the existing frame. This causes the glass area to shrink slightly to accommodate the new window. When installing a "new construction window", the entire window unit including sash and frame is removed down to the rough opening and the complete assembly is replaced which includes a nailing flange which has to be secured to the exterior wall. Installing new construction windows is a much more complex project since the siding on the exterior has to be removed in order to secure the nail flange to the wall. Most consumers go with the replacement window do to the ease and cost of installation as compared to the new construction window option. Hope this helps!

You can and you should replace your existing windows with new construction windows because that is the best method. A "replacement" window is really designed for an older home where the window sashes and jambliners are held in place by strips of wood called removable stops. When you remove the interior stop or cut out the exterior stop (to be replaced by a new piece of wood), you are able to remove the sashes and jambliner and slip in a "pocket" replacement window (vinyl, fiberglass or wood) and screw it in through the jamb.

The better option is to remove the entire window down to the studs and replace it with a new full-frame window. It is more labor-intensive and costs about $150 to $200 more per opening but there are some advantages: no loss in visible glass, the ability to insulate around the frame of the new window, and you get a better peek at the framing of the studs around the window. It also requires new interior trim, but that is also an advantage.

Having said the above, we've done a lot of work on older homes (1920's etc) with very ornate interior trim that we were reluctant to disturb and a pocket replacement window was the right way to go.

Inserts can be done very effectively with no weak links in the system in a vast majority of the cases. Where it really pays dividends to go with a full, new construction window is in situations where you are disturbing the drainage plane and not able to properly access and rebuild it via insert application

Abe's pictures are a perfect example of that.

If you can afford the cost differential and you are thinking about doing a siding project...absolutely do them together. That is the most comprehensive installation to go back with the proper flashing, nailing flange, drip cap, etc.